My idea is for a series of entities existing within the earth's mantle. These entities are highly intelligent and communicated telepathically with humans on a regular basis, although the Foundation has been able to limit it through anomalous technology to a few sites.

These entities are extremely ancient, believed to originate from the Hadean era on earth. They regularly toy with humans, viewing us as tools for their amusement. They've reached an agreement with humans long ago: Entertain us with this ritual at this interval or we will break out and destroy the world. They, of course, found a loophole and regularly create anomalous entities and species, as well as influence current events, since technically their agreement with humans allows this.

I dunno… it seems kind of bland. Why do these ancient things, given their intelligence and abilities, want to mess with humans in particular? Why do they live in the mantle rather than on the surface? What's with their obsession with a ritual? Also, if they're so old, have they been messing with humans since before civilization started?